The 725th episode of Doctor Who (the 27th since the series’ revival) airs on BBC1. Tracy-Ann Oberman guest stars. This is the conclusion of the second season of the new series, and sees Billie Piper’s exit as a series regular. The end of the episode introduces Catherine Tate as future companion Donna Noble, leading into the 2006 Christmas episode. This episode is the first to pit the Daleks against the Cybermen.